Carbon-carbon composite materials are known to exhibit excellent mechanical properties and structural integrity up to 2775 K. in a vacuum. In oxidative conditions, however, at temperatures above 1075 K., carbon-carbon composite materials are rapidly degraded.
A review of the coatings for carbon-carbon composite materials for oxidation resistance has been presented by Wittmer, D. T., "Protective Coatings for Carbon-Carbon Composites," Metal Matrix, Carbon and Ceramic Matrix Composites, NASA Conference Publication 2482, pp. 381-94 (1987). Wittmer identified the problems inherent with brittle SiC, TiC and Si.sub.3 N.sub.4 coatings for high temperature cyclic oxidation resistance. Silicon carbide coatings have been used to protect carbon-carbon composite materials from oxidation. However, such coatings and carbon-carbon composites exhibit different thermal expansion characteristics. These differences, and the resulting effects of temperature on coating adherence and strength, significantly reduce the applicability of silicon carbide coatings in applications involving thermal cycling. The use of low thermal expansion materials has been suggested by Kerans, R. J. et al., "Low Thermal Expansion Coatings for Carbon-Carbon Composites," Metal Matrix, Carbon and Ceramic Matrix Composites, NASA Publication 2482, pp. 409-25 (1987).
Literature also exists on a variety of coating systems involving laser processing of plasma-sprayed, ceramic materials. Iwamoto, N. et al., "Surface Treatment of Plasma-Sprayed Ceramic Coatings by a Laser Beam," Surface and Coatings Technology, vol. 34, no. 1, pp. 59-67 (1988), discusses laser treatment to densify plasma-sprayed alumina and zirconia coatings. Leong, K. et al., "Development of High Temperature Oxidation-Resistant Materials for Tactical Missiles," Report NSWC TR 87-244 for the Naval Surface Weapons Center (1986), discusses laser processing of plasma deposited hafnium diboride, hafnium carbide and zirconium carbide coatings. U.S. Pat. No. 4,814,232 to Bluege et al., describes a method of making a mirror, consisting of a readily polishable metal or ceramic intermediate layer applied to a mirror substrate by plasma spray techniques. The outer surface of the intermediate layer may be smoothed by a high energy beam. A reflective coating layer is then applied to the polished intermediate layer.
Despite the advancements in the art, there is still a need for dense, defect-free coatings, capable of protecting carbon-carbon composite materials under oxidative high temperature conditions and having adhesive properties and coefficients of thermal expansion that are compatible with carbon-carbon composite and graphitic materials, and efficient methods for fabricating such coatings.